Bac F1 Bénin common mistakes
Bac F1 Bénin · Understand the traps and how to avoid them.
Reading a dimension off a drawing by counting millimetres between lines instead of using the stated numerical value and scale, then reporting a length that contradicts the written cote.
Candidates treat the drawing as a picture to be measured rather than as a contract in which the written value governs; the habit comes from early exercises where the sketch was drawn to scale and the value was never given.
What to do instead
Adopt the rule 'the written value always wins' and annotate each cote with its tolerance before computing anything; when a value is missing, write 'valeur non cotée' and use the stated scale factor explicitly in the calculation, never an eyeballed measurement.
Confusing the three datum systems (système arbre, système alésage, mixed) when writing an adjustment: assigning an ISO tolerance letter to the hole that belongs to the shaft, or inverting the sign of the fundamental deviation.
The letter-to-position mapping is memorised as a list rather than understood as 'H means the hole is the reference, h means the shaft is the reference'; under exam pressure the letters blur.
What to do instead
Before writing any fit, state in one sentence which part is the reference and which is adjusted, then derive each deviation from that sentence; check by computing the clearance/interference numerically and confirming the sign matches the fit type (jeu, serrage, incertain).
Drawing a free-body diagram that omits a reaction (a roller support drawn as a pin, or an internal force left in) and then solving equilibrium with the wrong number of unknowns.
Candidates copy the symbol from the sketch without translating it into its mechanical meaning, and they start writing equations before the diagram is complete.
What to do instead
Build the diagram in a fixed order (isolate the body, mark known loads, then one reaction per removed liaison), count unknowns against the three equilibrium equations, and only then write the equations; if unknowns exceed equations, flag the structure as hyperstatic rather than forcing a solution.
Applying the wrong bending-moment formula or forgetting the section's own inertia, using a diameter where a radius is required (or using d instead of d⁴/64) in the RDM calculation.
Formulas are learned as strings without the geometric meaning of each term, and the moment of inertia of a circular section is the single most frequently mis-remembered quantity.
What to do instead
Write the symbol legend under every formula before substituting (I, v, Mf, σ), compute I separately and check its unit (mm⁴ or m⁴), and verify the final stress is in MPa with a plausible order of magnitude for steel (tens of MPa, not thousands).
Treating a kinematics problem as if acceleration were constant when the motion is circular or the speed varies, and applying v = v₀ + at to a rotating shaft or a decelerating spindle without checking the hypothesis.
The uniform-acceleration formulas are the only ones practised intensively, so they are applied reflexively to any motion problem.
What to do instead
Before choosing a formula, write the nature of the motion (uniform, uniformly accelerated, circular) and the quantity that is constant; for rotation use ω, α and the angular relations, and for circular motion keep the normal acceleration v²/R in the vector sum.
Choosing cutting parameters by copying a neighbour's values or by reflex (e.g. always 0.2 mm/rev, always 800 rpm) without linking speed, feed and depth of cut to the material, tool grade and operation type.
Shop-floor habits and remembered numbers replace the catalogue reasoning; candidates lack a structured method for reading a cutting-condition table.
What to do instead
State the material, the tool material and the operation (ébauche/finition) first, read Vc from the table, then compute N = 1000·Vc/(π·D) explicitly, and justify the feed by the required roughness or the finishing pass; show the three parameters as a justified triple, not three isolated numbers.
Reading a vernier or micrometer to the wrong resolution: taking the vernier's 1/50 mm as 1/20, or reading the thimble of a micrometer in hundredths when the sleeve already shows half-millimetres.
The reading procedure is done by imitation without understanding that resolution = smallest division of the main scale divided by the number of vernier divisions; the half-millimetre mark on the sleeve is easily missed.
What to do instead
For every instrument, first write its resolution and the reading rule in one line, then read main scale, then vernier/thimble, then add; practise by checking that a measured value on a known gauge block falls within the instrument's stated uncertainty.
Manipulating algebraic expressions with units and trigonometric functions without checking dimensional homogeneity, e.g. adding a length to an area or writing sin of a quantity that is not an angle.
Mathematics is studied separately from mechanics, so the habit of ignoring units during manipulation carries over into applied problems.
What to do instead
Carry units through every line of the calculation, convert everything to SI at the start, and finish each derivation with a homogeneity check; if the two sides of an equation do not share a unit, stop and locate the error before continuing.
Mixing sign conventions and unit systems in electricity and thermodynamics: using degrees Celsius in a gas-law ratio, forgetting the sign of work in the first law, or omitting the internal resistance of a source when applying Ohm's law to a real circuit.
Each sub-topic is revised in isolation, and the conventions (ΔU = Q + W versus Q − W, generator versus receiver) are not written down at the start of the exercise.
What to do instead
Open every physics answer by writing the convention chosen and converting temperatures to kelvin; draw the circuit with the source's internal resistance included, and check the energy balance numerically at the end (sum of powers zero, or Q and W consistent with ΔU).
Answering a technical-communication question with an unstructured paragraph: no identified receiver, no functional analysis of the need, and a report or notice that mixes description, justification and instructions in one block.
Candidates write as they speak and are not trained to separate the functions of a technical text (informing, prescribing, justifying); the time pressure encourages a single draft.
What to do instead
Before writing, fill a three-line plan (who reads it, what they must know, what they must do), then produce the text with a heading per function, short sentences in the present or imperative as appropriate, and a final reread checking terminology, agreement and the presence of units and references.